Semiconductor memory device

ABSTRACT

A semiconductor memory device comprises a plurality of memory layers arranged in multilayer, each memory layer including a cell array, the cell array containing a plurality of first parallel lines, a plurality of second parallel lines arranged crossing the first lines, and a plurality of memory cells connected at intersections of the first lines and the second lines; a pulse generator operative to generate pulses required for data access to the memory cell; and a control means operative to control the pulse generator such that the pulse output from the pulse generator has energy in accordance with the memory layer to which the access target memory cell belongs.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/394,712 filed Feb. 27, 2009, entitled “Semiconductor Memory Device,”which is based upon and claims the benefit of priority from the priorJapanese Patent Application No. 2008-50626, filed on Feb. 29, 2008, theentire contents of both of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory device, and moreparticularly to a stack-layer-structured semiconductor memory deviceusing variable resistors.

2. Description of the Related Art

Electrically erasable programmable nonvolatile memories include a flashmemory as well known in the art, which comprises a memory cell array ofNAND-connected or NOR-connected memory cells having a floating gatestructure. A ferroelectric memory is also known as a nonvolatile fastrandom access memory.

On the other hand, technologies of patterning memory cells much finerinclude a resistance variable memory, which uses a variable resistor ina memory cell as proposed. Known examples of the variable resistorinclude a phase change memory element that varies the resistance inaccordance with the variation in crystal/amorphous states of achalcogenide compound; an MRAM element that uses a variation inresistance due to the tunnel magneto-resistance effect; a polymerferroelectric RAM (PFRAM) memory element including resistors formed of aconductive polymer; and a ReRAM element that causes a variation inresistance on electrical pulse application (Patent Document 1: JP2006-344349A, paragraph 0021).

The resistance variable memory may configure a memory cell with a serialcircuit of a Schottky diode and a resistance variable element in placeof the transistor. Accordingly, it can be stacked easier andthree-dimensionally structured to achieve much higher integrationadvantageously (Patent Document 2: JP 2005-522045A).

A semiconductor memory device having a stack-layered structure comprisesmemory layers, which differ in thermal history resulted from the processof stacking layers. Namely, the lower the memory layer, the more it issusceptible to heat . As a result, the oxidation degrees of the metaloxide vary among the memory layers and cause differences in writecharacteristic among the memory cells.

SUMMARY OF THE INVENTION

In an aspect the present invent ion provides a semiconductor memorydevice, comprising: a plurality of memory layers arranged in multilayer,each memory layer including a cell array, the cell array containing aplurality of first parallel lines, a plurality of second parallel linesarranged crossing the first lines, and a plurality of memory cellsconnected at intersections of the first lines and the second lines; apulse generator operative to generate pulses required for data access tothe memory cell; and a control means operative to control the pulsegenerator such that the pulse output from the pulse generator has energyin accordance with the memory layer to which the access target memorycell belongs.

In another aspect the present invention provides a semiconductor memorydevice, comprising: a plurality of memory layers arranged in multilayer,each memory layer including a cell array, the cell array containing aplurality of first parallel lines, a plurality of second parallel linesarranged crossing the first lines, and a plurality of memory cellsconnected at intersections of the first lines and the second lines, eachmemory cell containing an electrically rewritable variable resistoroperative to store the resistance as data; a pulse generator operativeto generate pulses required for data access to the memory cell ; and acontrol means operative to control the pulse generator such that thepulse output from the pulse generator has energy in accordance with thememory layer to which the access target memory cell belongs.

In yet another aspect the present invention provides a semiconductormemory device, comprising: a cell array containing a plurality of firstparallel lines, a plurality of second parallel lines arranged crossingthe first lines, and a plurality of memory cells connected atintersections of the first lines and the second lines; a pulse generatoroperative to generate pulses required for data access to the memorycell; and a control means operative to control the pulse generator suchthat the pulse output from the pulse generator has energy in accordancewith the access target memory cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a semiconductor memory device according toa first embodiment of the present invention.

FIG. 2 is a perspective view of part of a memory cell array in thesemiconductor memory device according to the same embodiment.

FIG. 3 is a cross-sectional view of one memory cell taken along I-I′line and seen from the direction of the arrow in FIG. 2.

FIG. 4 is a schematic cross-sectional view showing a variable resistorexample in the same embodiment.

FIG. 5 is an equivalent circuit diagram of the memory cell array in thesemiconductor memory device according to the same embodiment.

FIG. 6 is a perspective view showing part of the memory cell array inthe semiconductor memory device according to the same embodiment.

FIG. 7 is a cross-sectional view of the semiconductor memory deviceaccording to the same embodiment.

FIG. 8 is a brief diagram of writing in the semiconductor memory deviceaccording to the same embodiment.

FIG. 9 is a graph showing a first example of pulse voltages forrealizing writing of FIG . 8 in the semiconductor memory deviceaccording to the same embodiment.

FIG. 10 is a graph showing a second example of pulse voltages forrealizing writing of FIG . 8 in the semiconductor memory deviceaccording to the same embodiment.

FIG. 11 is a graph showing a third example of pulse voltages forrealizing writing of FIG . 8 in the semiconductor memory deviceaccording to the same embodiment.

FIG. 12 is a brief diagram of writing with electrical energy stepped upseveral times in the semiconductor memory device according to the sameembodiment.

FIG. 13 is a graph showing pulse voltages for realizing writing of FIG.13 in the semiconductor memory device according to the same embodiment.

FIG. 14 is a brief diagram of writing with electrical energy steppeddown several times in the semiconductor memory device according to thesame embodiment.

FIG. 15 is a graph showing an example of pulse voltages for realizingwriting of FIG. 14 in the semiconductor memory device according to thesame embodiment.

FIG. 16 is an equivalent circuit diagram of a memory cell and lines in asemiconductor memory device according to a second embodiment of thepresent invention.

FIG. 17 is a graph showing relations between voltage drops andresistances of word lines in array layers in the semiconductor memorydevice according to the same embodiment.

FIG. 18 is a graph showing an example of pulse voltages in thesemiconductor memory device according to the same embodiment.

FIG. 19 is a circuit diagram showing part of a semiconductor memorydevice according to a third embodiment of the present invention.

FIG. 20 provides a graph showing an example of pulse voltages in thesemiconductor memory device according to the same embodiment.

FIG. 21 is a graph showing another example of pulse voltages in thesemiconductor memory device according to the same embodiment.

FIG. 22 provides a graph showing an example of pulse voltages in asemiconductor memory device according to a fourth embodiment of thepresent invention.

FIG. 23 provides a graph showing an example of pulse voltages in asemiconductor memory device according to a fifth embodiment of thepresent invention.

FIGS. 24A and 24B provide graphs showing an example of pulse voltages ina semiconductor memory device according to a sixth embodiment of thepresent invention.

FIG. 25 is a circuit diagram showing a current limiter circuit in asemiconductor memory device according to a seventh embodiment of thepresent invention.

FIGS. 26A and 26B provide graphs showing an example of pulse voltagesand limit values of current flowing in memory cells in the semiconductormemory device according to the seventh embodiment of the presentinvention.

FIG. 27 is an equivalent circuit diagram of a memory cell and lines in asemiconductor memory device according to yet another embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The embodiments associated with the semiconductor memory deviceaccording to the present invention will now be described with referenceto the drawings.

First Embodiment

FIG. 1 is a block diagram of a semiconductor memory device according toa first embodiment of the present invention.

The semiconductor memory device comprises a memory cell array 1 ofmemory cells arranged in matrix, each memory cell including alater-described ReRAM (variable resistor). A column control circuit 2 isprovided on a position adjacent to the memory cell array 1 in the bitline BL direction. It controls the bit line BL in the memory cell array1 to erase data from the memory cell, write data in the memory cell, andread data out of the memory cell. A row control circuit 3 is provided ona position adjacent to the memory cell array 1 in the word line WLdirection. It selects the word line WL in the memory cell array 1 andapplies voltages required to erase data from the memory cell, write datain the memory cell, and read data out of the memory cell.

A data I/O buffer 4 is connected to an external host, not shown, via anI/O line to receive write data, receive erase instructions, provide readdata, and receive address data and command data. The data I/O buffer 4sends received write data to the column control circuit 2 and receivesread-out data from the column control circuit 2 and provides it toexternal. An address fed from external to the data I/O buffer 4 is sentvia an address register 5 to the column control circuit 2 and the rowcontrol circuit 3. A command fed from the host to the data I/O buffer 4is sent to a command interface 6. The command interface 6 receives anexternal control signal from the host and decides whether the data fedto the data I/O buffer 4 is write data, a command or an address. If itis a command, then the command interface transfers it as a receivedcommand signal to a state machine 7. The state machine 7 manages theentire semiconductor memory device to receive commands from the host,read, write, erase, and execute data I/O management. The external hostcan also receive status information managed by the state machine 7 anddecide the operation result . The status information is also utilized incontrol of write and erase.

The state machine 7 controls the pulse generator 8 . Under this control,the pulse generator 8 is allowed to provide a pulse of any voltage atany timing. Specifically, the state machine 7 receives an address fedfrom external via the address resistor 5 to determine which memory layeris the access target, and uses a parameter corresponding to that memorylayer to control the height and width of the pulse from the pulsegenerator 8. This parameter: is a value derived by graspingcharacteristics of write and so forth in the memory layers to even writecharacteristics in the memory layers, and saved among the memory cells.The pulse formed herein can be transferred to any line selected by thecolumn control circuit 2 and the row control circuit 3.

Peripheral circuit elements other than the memory cell array 1 can beformed in a silicon substrate immediately beneath the memory array 1.Thus, the chip area of the semiconductor memory device can be madealmost equal to the area of the memory cell array 1.

FIG. 2 is a perspective view of part of the memory cell array 1, andFIG. 3 is a cross-sectional view of one memory cell taken along I-I′line and seen in the direction of the arrow in FIG. 2.

There are plural first lines or bit lines BLO-BL2 disposed inparallel,which cross plural second lines or word lines WLO-WL2 disposed inparallel. A memory cell MC is arranged at each intersection of bothlines as sandwiched therebetween. Desirably, the first and second linesare composed of heat-resistive low-resistance material such as W, WSi,NiSi, CoSi.

The memory cell MC comprises a serial connection circuit of a variableresistor VR and a non-ohmic element NO as shown in FIG. 3.

The variable resistor VR can vary the resistance through current, heat,or chemical energy on voltage application. Arranged on an upper and alower surface thereof are electrodes EL2, EL1 serving as a barrier metaland an adhesive layer. Material of the electrodes may include Pt, Au,Ag, TiA1N, SrRuO, Ru, RuN, Ir, Co, Ti, TiN, TaN, LaNiO, Al, PtIrOx,PtRhOx, Rh/TaA1N. A metal film capable of achieving uniform orientationmay also be interposed. A buffer layer, a barrier metal layer and anadhesive layer may further be interposed.

The variable resistor VR may be one that includes a material such aschalcogenide that varies its resistance through the phase change betweenthe crystal state and the amorphous state (PCRAM); may be one thatvaries its resistance through precipitation of metal cations to form abridge (conducting bridge) between electrodes or ionize the precipitatedmetal to break the bridge (CBRAM); and may be one that varies itsresistance through voltage or current application (ReRAM) (which isroughly divided into two: one that causes a resistance variation inresponse to the presence/absence of charge trapped in a charge trappresent in an electrode interface, and another that causes a resistancevariation in response to the presence/absence of a conduction path dueto a loss in oxygen).

FIG. 4 shows an example of the variable resistor VR comprising ReRAM.The variable resistor VR shown in FIG. 4 includes a recording layer 10arranged between electrode layers 9, 11 . The recording layer 10 iscomposed of a composite compound containing at least two types of cationelements. At least one of the cation elements is a transition elementhaving the d-orbit incompletely filled with electrons, and the shortestdistance between adjacent cation elements is 0.32 nm or lower.Specifically, the variable resistor VR is represented by a chemicalformula A_(X)M_(Y)X_(Z) (A and M are different elements) and may beformed of material having a crystal structure such as a spinel structure(AM₂O₄) , an ilmenite structure (AMO₃) , a delafossite structure (AMO₂), a LiMoN₂ structure (AMN₂) , a wolframite structure (AMO₄), an olivinestructure (A₂MO₄), a hollandite structure (A_(x)MO₂), a ramsdellitestructure (A_(x)MO₂) , and a perovskite structure (AMO₃).

In the example of FIG. 4, A comprises Zn, M comprises Mn, and Xcomprises O. In the recording layer 10, a small white circle representsa diffused ion (Zn), a large white circle represents an anion (O), and asmall black circle represents a transition element ion (Mn) . Theinitial state of the recording layer 10 is the high-resistance state.When the electrode layer 9 is kept at a fixed potential and a negativevoltage is applied to the electrode layer 11, part of diffused ions inthe recording layer 10 migrate toward the electrode layer 11 to reducediffused ions in the recording layer 10 relative to anions. The diffusedions arrived at the electrode layer 11 accept electrons from theelectrode layer 11 and precipitate as a metal, thereby forming a metallayer 12. Inside the recording layer 10, anions become excessive andconsequently increase the valence of the transition element ion in therecording layer 10. As a result, the carrier injection brings therecording layer 10 into electron conduction and thus completes setting.On regeneration, a current may be allowed to flow, of which value isvery small so that the material configuring the recording layer 10causes no resistance variation. The programmed state (low-resistancestate) may be reset to the initial state (high-resistance state) bysupplying a large current flow in the recording layer 10 for asufficient time, which causes Joule heating to facilitate the oxidationreduction reaction in the recording layer 10. Application of an electricfield in the opposite direction from that at the time of setting mayalso allow resetting.

FIG. 5 is an equivalent circuit diagram of a memory cell array of 2×2 inthe semiconductor memory device according to the present embodimentusing the variable resistor VR shown in FIG. 4. As shown, accessibleelements, for example, resistance variable memory cells MC, eachincluding a diode Di and a variable resistor VR serially connected, arearranged at intersections of the word lines WL and the bit lines BL.

In the circuitry of FIG. 5, the word lines WL are usually at “L” (0V)and the bit lines BL at “H” (Vp) . When the word line WLO is turned from“L” to “H” and the bit line BL0 from “H” to “L”, the diode Di in thememory cell MC shown with A in FIG. 5 is forward biased and accordinglycurrent flows in the direction of the dotted line arrow. The variableresistor VR is in the low-resistance state or in the high-resistancestate as described above. Therefore, sensing that the current is largeor small enables data in the memory cell MC, A to be read out.

If the pulse voltage Vp applied to the word line WLO is elevated to avoltage required for set and reset, it can set and reset the variableresistor VR in the memory cell MC, A.

FIG. 6 is a perspective view showing part of the memory cell array inthe semiconductor memory device according to the same embodiment. Thispart includes plural memory layers CA stacked, each including the arrayof memory cells shown in FIG. 5.

The example herein shown includes four memory layers CA0-CA3 stacked ona silicon substrate 13. The bit lines BL in the memory layers CA areconnected in common through via-lines 16 to a column control circuit 15on the substrate 13 . The word lines WL located on the memory layersCA0-CA3 are independently connected through via-lines 17 to a rowcontrol circuit 14 on the silicon substrate 13.

FIG. 7 is a cross-sectional view of the semiconductor memory devicehaving the above-described stack-layered structure. There is provided asilicon substrate 18 having a well 19 formed therein, on which animpurity-diffused layer 20 and a gate electrode 21 of a transistorcontained in a peripheral circuit are formed. A first interlayerinsulator 22 is deposited thereon. A via-hole 23 is formed appropriatelythrough the first interlayer insulator 22 to the surface of the siliconsubstrate 18. A first metal 24 configuring the first line or bit line BLin the memory cell array is formed of a low-resistance metal such as Won the first interlayer insulator 22. A barrier metal 25 is formed in anupper layer above the first metal 24. The barrier metal may be formed ina lower layer below the first metal 24. The barrier metal may be formedof both or one of Ti and TiN. A non-ohmic element 26 such as a diode isformed above the barrier metal 25. On the non-ohmic element 26, a firstelectrode 27, a variable resistor 28 and a second electrode 29 areformed in this turn, thereby configuring a memory cell MC including thebarrier metal 25 through the upper electrode 29. A barrier metal may beinterposed below the first electrode 24 and above the second electrode29. A barrier metal and an adhesive may be interposed below the secondelectrode 29 and above the lower electrode 24. A second interlayerinsulator 30 and a third interlayer insulator (not shown) are buriedbetween the memory cell MC and an adjacent memory cell MC. On the memorycells MC in the memory cell array, a second metal 31 is formed toconfigure a second line or word line WL extending in the directionperpendicular to the bit line BL. A fourth interlayer insulator 32 isformed thereon. A multi-layered structure may be realized by stackingthe first metal 24 through the fourth interlayer insulator 32 andforming the second interlayer insulator 30 and the third interlayerinsulator between the memory cells MC, repeatedly by the number oflayers required.

A method of manufacturing the semiconductor memory device according tothe present embodiment shown in FIG. 7 is described next.

First, a FEOL (Front End of Line) process for forming transistors and soforth to form necessary peripheral circuits on the silicon substrate 18is executed, and then the first interlayer insulator 22 is depositedthereon. The via-hole 23 is formed as well in this step.

Subsequently, the upper layer portion above the first metal 24 isformed.

Once the first interlayer insulator 22 and the via--hole 23 are formedas described above, deposition thereon of a layer turned into the firstmetal 24 in the memory cell array, formation of a layer turned into thebarrier metal 25, deposition of a layer turned into the non-ohmicelement 26, deposition of a layer turned into the first electrode 27,deposition of a layer turned into the variable resistor 28, anddeposition of a layer turned into the second electrode 29 are executedsequentially. Through the above steps, the stacked body of the upperlayer portion can be formed.

Subsequently, a hard mask of TEOS or the like, not shown, is formed onthe upper surface of the stacked body, and a first anisotropic etchingis executed with this mask to form trenches along the bit line BL toseparate the stacked body.

Next, the second interlayer insulator is buried in this trench. For thesecond interlayer insulator, a suitable material has excellentinsulation, a low capacity and an excellent burial property.Subsequently, a process of CMP or the like is applied in planarizationto remove extra portions from the second interlayer insulator 30 andexpose the upper electrode 29.

A layer of W or the like turned into the second metal 31 is stacked overthe planarized portion after CMP. Thereafter, a hard mask of TEOS or thelike is formed on this layer, and a second etching is executed with L/S(Line and Space) in the direction crossing the first etching, therebyforming trenches along the word line orthogonal to the bit line BL. Atthe same time, the memory cells MC separated in pillar shapes are formedat cross-points of the bit lines BL and the word lines WL in aself-aligned manner. Subsequently, the third interlayer insulator 30 isburied and then the third interlayer insulator 30 is planarized, therebyforming the memory array layer of the cross-point type.

Thus , through stacking flat films and patterning them twice withorthogonal L/S, such the cross-point cells can be formed in aself-aligned manner without any misalignment with lines. The formationof the above stack-layered structure can be repeated to form the memorycell array of the multi-layered cross-point type.

During the formation of the memory cell array through the above process,film formation and formation of protective films produce a large amountof heat . Therefore, when a certain memory layer CA is stacked, theassociated thermal history exerts the influence on lower memory layersCA and wiring layers.

The control means of the present embodiment compensates for differencesin resistance in the initial state among the memory cells MC due to suchthe differences in thermal history of the memory layers CA, fordifferences in characteristic among non-ohmic elements and in resistanceamong lines, or for differences in write, erase and read characteristic.For that purpose, the control means controls the pulse generator 7 tochange formation of pulse voltages applied on writing, erasing andreading at every memory layer CA.

The pulses controlled by the pulse generator 7 are described next withspecific examples shown.

FIG. 8 is a brief diagram of writing in the memory layers CA in thesemiconductor memory device according to the first embodiment of thepresent invention.

FIG. 8 shows resistances in the initial state of the memory cells MCbelonging to the lowermost memory layer CA0 through the uppermost memorylayer CA3. As can be found, the lower the layer, the higher theresistance becomes. This is because the lower the layer, the more thethermal histories , and the influence thereby changes the oxidationdegree of the metal oxide and increases the resistance of the variableresistance material of the memory cell MC. As a result, the lower thelayer, the worse the write characteristic for the transition to theprogrammed state becomes and the larger the electrical energy requiredfor writing becomes as shown by the arrow in FIG. 8.

Examples of the pulse voltage Vp under control of the pulse generator 7on writing are shown in FIGS. 9-11.

FIG. 9 shows an example of the semiconductor memory device, which cannot apply different pulse voltages Vp to the memory arrays CA at thesame time. In this case, the height of the pulse voltage Vp is changedat each memory layer CA, thereby controlling electrical energy fed tothe memory cell MC in each memory layer CA.

It is assumed now that data write occurs sequentially from the memorycell belonging to the uppermost memory layer CA3 to the memory cellbelonging to the lowermost memory layer CA0. Initially, the statemachine 7 accesses the memory cell array 1 to read a parameterspecifying a pulse height on writing in each of the memory layersCA0-CA3, and stores the parameter in an inner resister or sets it in thepulse generator 8. Then, the state machine 7 uses the write address toindentify the write target layer with the memory layer CA3 and controlsthe pulse generator 8 based on the parameter to generate a pulse havingthe magnitude in accordance with the memory layer CA3. Thus, the pulsegenerator 8 generates a pulse voltage Vp3 with the least energy. Thepulse voltage Vp3 is applied as the program voltage Vp to the selectedword line WL in the memory layer CA3. Thereafter, the height of thepulse voltage applied is increased sequentially toward the lowermostmemory layer CA0, thereby evening the write characteristics among thememory cells in each of the memory layers.

FIG. 10 shows an example of changing not the height but the width of thepulse voltage, thereby controlling the electrical energy fed to thememory cells MC in each memory layer CA.

Thus, the lower the memory layer CA, the longer the pulse width appliedis made to compensate for the write characteristic in each memory layerCA, which worsens in a lower layer than an upper layer, thereby eveningthe write characteristics among all the memory layers CA.

FIG. 11 shows an example of applying different pulse voltages to all thememory layers CA at the same time.

FIG. 11 shows application of pulse voltages different in height to thememory cells MC in the memory layers CA at the same time. In this case,naturally, faster writing can be achieved than those in FIGS. 9 and 10.

The following description is given to an example of realizing writingwith the pulse voltage Vp applied several times.

As shown in FIG. 12, with writing executed in stages from the initialstate to the programmed state, a sharp increase in current can beprevented from arising when the resistance of the variable resistor VRvaries.

FIG. 13 shows an example of the pulse voltages Vp under control of thepulse generator 7 for realizing writing shown in FIG. 12.

The pulse voltages different in height are applied to the lowermostmemory layer CA0 through the uppermost memory layer CA3 like in theexample of FIG. 11. Different from the example of FIG. 11, however, inthe example of FIG. 13, the pulse generator 7 is controlled to apply arelatively low pulse voltage Vp to each memory layer CA and thenrepeatedly apply the pulse voltage Vp to the memory layers CA withincreases in step width.

Writing may be processed faster with an extended step width.

On the other hand, the closer the state to the programmed one, thesmaller the electrical energy fed may be made as shown in FIG. 14 torealize writing. Thus, it is possible to finely adjust the resistance asthe state is closer to the programmed one, thereby achieving a narrowerdistribution of resistances of the memory cells MC in the programmedstate.

FIG. 15 shows an example of pulse voltages for realizing writing shownin FIG. 14.

In the example of FIG. 15, required electrical energy is fed in stageslike in FIG. 13, though, the pulse generator 7 is controlled torepeatedly apply the pulse voltage Vp to the memory layers CA withdecreases in step width.

In the present embodiment, differences in characteristic of lines,non-ohmic elements and variable resistance memory elements caused by theinfluence through the process of stacking layers can be evened byoptimizing voltages and widths of write, erase and read pulses at eachmemory layer, thereby evening write, erase and read characteristicsamong the memory layers.

Second Embodiment

In the first embodiment, the differences in characteristic of variableresistors among the different memory layers are considered.

It is also required, however, to consider the differences incharacteristic of non-ohmic elements, electrodes and wiring layers otherthan the variable resistors.

This is because the non-ohmic elements NO cause differences in impuritydiffusion among the memory layers CA due to the influence by thermalhistories, which cause variations in resistance, threshold, breakdownvoltage and so forth while the electrodes and wiring layers causedifferences in resistance due to the influence by oxidation and soforth.

Specifically, it is described with the use of FIG. 16.

FIG. 16 is an equivalent circuit diagram of a memory cell and lines inthe semiconductor memory device according to the present embodiment.

The memory cell MC includes a diode Di and a variable resistor VRserially connected to the diode Di on the cathode. The anode of thediode Di is connected to the word line WL. The other end of the variableresistor VR is connected to the bit line BL that is connected to theground line Vss.

When a certain voltage V is applied to the word line WL in this circuit,the voltage Vcell across the memory cell MC is made equal to the voltageV minus (the voltage drop Vwl on the word line WL plus the voltage dropVbl on the bit line BL).

Relations between the voltage drop Vwl and the resistance of the wordline WL in the memory layers are shown in FIG. 17. As described above,the lower the memory layer CA, which is greatly susceptible to theinfluence by heat through the process of stacking layers, the higher theresistance of the word line WL, which increases the voltage drop Vwlaccordingly, as canbe found. Therefore, even if the same voltage V isapplied to the memory layers, the voltage Vcell across the memory cellMC becomes smaller in a memory cell MC connected to the word line WL ina lower layer.

As a result, the differences in the voltage drop Vwl on the word line WLlead to the differences in the voltage Vcell across the memory cell MC,and thus cause the differences in write, erase and read characteristicsamong the memory layers.

Write pulses for compensating for the differences in characteristicsamong the memory layers to even the characteristics are shown in FIG.18.

As shown in FIG. 18, a higher pulse is applied to the word line WLbelonging to a lower memory layer, thereby making the voltage Vcellacross the memory cell MC at a constant voltage.

In the present embodiment, differences in characteristic of lines andnon-ohmic elements in the stack-layer-structured semiconductor memorydevice using variable resistors caused by the influence through theprocess of stacking layers canbe evened by optimizing voltages andwidths of write, erase and read pulses at each memory layer. Thus, it ispossible to provide a semiconductor memory device with evened write,erase and read characteristics among the memory layers.

Third Embodiment

The first and second embodiments consider variations in characteristicdue to differences in thermal history and apply different pulse voltagesto the memory layers correspondingly to even the characteristics amongthe memory layers.

Even the same memory layer, however, causes a voltage drop and a CRdelay due to the parasitic resistances and capacitances on the word andbit lines. Accordingly, variations may arise in characteristic dependingon the place of the memory cell. This causes a problem in particularwhen the memory layer has a larger size.

In a semiconductor memory device according to a third embodiment of thepresent invention, different pulse voltages are applied to the memorycells correspondingly in writing based on the address.

FIG. 19 is a circuit diagram showing part of the semiconductor memorydevice according to the present embodiment.

This semiconductor device comprises a cell array, which contains wordlines WL0-WL2, bit lines BL0-BL2 crossing these word lines WL, andmemory cells MC0-MC8 provided at intersections of the word lines WL andthe bit lines BL. A column control circuit 2 is arranged on one end ofthe bit line BL close to the word line WL2, and a row control circuit 3on one end of the word line WL close to the bit line BL0.

FIG. 20 shows pulse voltages Vp6, Vp0, Vp1 andVp2 on writing to thememory cells MC6, MC0, MC1 and MC2 shown in FIG. 19. In this example,the pulse voltages Vp are different in height among the memory cells MC.

Specifically, the lowermost pulse voltage Vp6 is applied to the memorycell MC6 located close to the means for supplying the pulse voltages Vpto the bit line BL and the word line WL, that is, closest to the columncontrol circuit 2 and the row control circuit 3, and gradually higherpulse voltages Vp0, Vp1 and Vp2 are applied to the memory cells MC0, MC1and MC2 arranged in order of location far from the column controlcircuit 2 and the row control circuit 3.

FIG. 21 shows an example of pulse voltages Vp with pulse widths variedamong the memory cells MC such that pulse voltages Vp6, Vp0, Vp1 and Vp2having gradually wider pulse widths are applied to the memory cells MC6,MC0, MC1 and MC2 in order.

As described above, in the present embodiment, pulse voltages Vpdifferent in height or width are applied to the memory cells MC tocompensate for variations in write characteristic due to the parasiticresistances and capacitances on the word line WL and bit line BL.Therefore, it is possible to provide a semiconductor memory device withevened write characteristics among the memory cells MC.

In the above description, different pulse voltages Vp are applied to thememory cells MC though different pulse voltages may be applied on agroup basis if plural neighbor memory cells MC are grouped.

Fourth Embodiment

In writing, a sharply rising pulse voltage Vp may be applied to thememory cell MC. In such the case, under the influence of the parasiticcapacitance on the diode Di contained in the memory cell MC, it may bereverse-biased, thereby causing a disturb problem. To suppress this,making slower the rise of the pulse voltage Vp is effective. With thisregard, in the case of writing to the memory cell MC located far fromthe column control circuit 2 and the row control circuit 3, the accesspath to that memory cell MC has an originally large parasitic resistanceand capacitance, which makes slower the rise and fall of the pulsevoltage Vp accordingly. On the other hand, in the case of the memorycell MC located close to the column control circuit 2 and the rowcontrol circuit 3, the pulse voltage Vp has a sharp rise and fall, whichmay cause a larger disturb risk accordingly.

In a semiconductor memory device according to a fourth embodiment of thepresent invention, a pulse voltage Vp with a slower rise and fall isapplied to the memory cell MC located close to the column controlcircuit 2 and the row control circuit 3.

FIG. 22 shows pulse voltages Vp6, Vp0, Vp1 and Vp2 applied on writing tothe memory cells MC6, MC0, MC1 and MC2 shown in FIG. 19 in the presentembodiment.

As can be seen from FIG. 22, the pulse voltage Vp6 with the slowest riseand fall is applied to the memory cell MC6 located closest to the columncontrol circuit 2 and the row control circuit 3, and the pulse voltagesVp0, Vp1 and Vp2 with gradually sharper rise and fall are applied to thememory cells MC0, MC1 and MC2 arranged in order of location far from thecolumn control circuit 2 and the row control circuit 3.

As described above, in the present embodiment, the rise and fall of thepulse voltage Vp applied to the memory cell MC is made slower tosuppress occurrences of disturb. In addition, pulse voltages Vpdifferent in rise and fall time are applied to the memory cells MCcorrespondingly to even write characteristics among the memory cells MC.

The rise and fall time of the pulse voltage Vp can be adjusted among thememory cells MC as described above. Alternatively, it may be adjusted ona group basis if plural neighbor memory cells MC are grouped, or on amemory-layer basis.

Fifth Embodiment

A semiconductor memory device according to a fifth embodiment of thepresent invention applies pulse voltages several times to execute datawrite in stages and applies pulse voltages different in starting valueof electrical energy to the memory cells correspondingly.

Specifically, a pulse voltage with a smaller starting value ofelectrical energy is applied to a memory cell excellent in writecharacteristic and a pulse voltage with a larger starting value ofelectrical energy to a memory cell poor in write characteristic.Thereafter, plural pulse voltages are applied while increasingelectrical energy with a certain step width.

FIG. 23 provides a graph showing relations between time and pulsevoltages Vp6, Vp0, Vp1 and Vp2 on writing to the memory cells MC6, MC0,MC1 and MC2 shown in FIG. 19. In this example, electrical energy isadjusted by the height of the pulse voltage Vp.

Sequentially from the memory cell MC6 located closest to the columncontrol circuit 2 and the row control circuit 3 toward the memory cellMC2 located farthest therefrom, different pulse voltages Vp havinggradually higher heights are applied, like in FIG. 20. In the example ofFIG. 23, however, a relatively low pulse voltage Vp is applied to eachmemory cell MC and then the pulse voltage Vp with a gradually increasedstep width is applied repeatedly to the memory cell MC, different fromthe example of FIG. 20.

As described, execution of writing to each memory cell MC through pluralpulse voltage applications makes it possible to prevent a sharp increasein current due to the variation in resistance of the variable resistorVR in each memory cell MC, like in the case of FIG. 12. Further, it ispossible to reduce the variation in write characteristic caused by theposition of the memory cell MC and make evener the write characteristics.

Not only the height of the pulse voltage Vp but also the step width maybe changed on a memory cell MC basis.

Sixth Embodiment

A sixth embodiment of the present invention is similar to the fifthembodiment in execution of programming to the memory cell in stagesexcept that verify is performed between steps and the result is used toadjust the electrical energy of the pulse voltage Vp applied at the nextstep.

FIG. 24A provides a graph showing a relation between time and voltage ofthe pulse voltage Vp6 applied to the memory cell MC6 shown in FIG. 19.

As shown in FIG. 24A, initially, a pulse voltage Vp6 having a certainpulse width is applied to the memory cell MC6 and then verifying isperformed to read out the state of the memory cell MC6. As a result, ifno data is programmed in the memory cell MC6, then a pulse voltage Vp6with a wider pulse width is applied. Thereafter, verifying, adjustingthe pulse width based on the verify result, and programming are repeateduntil data is normally programmed in the memory cell MC6.

FIG. 24B provides a graph showing a relation between time and voltage ofthe pulse voltage Vp2 applied to the memory cell MC2, which is poorer inwrite characteristic than the memory cell MC6 and designed to have awider initial pulse width than the memory cell MC6.

FIGS. 24A and 24B both show the examples of widening the pulse width inaccordance with the verify result though the pulse width may be narrowedconversely. In this case, the state of the variable resistor VR in thememory cell MC can be made closer to a desired state with high accuracy.

Seventh Embodiment

When data program is performed in stages as in the sixth embodiment, thecurrent flowing in the memory cell MC increases during the transition ofthe variable resistor VR in the memory cell MC from the high-resistancestate to the low-resistance state, and accordingly it may destruct thememory cell MC possibly. With this regard, a limit imposed on thecurrent flowing in the memory cell MC can reduce the possibility ofdestruction of the memory cell MC. If the current is limited, however,in the memory cell MC poor in write characteristic in particular, datawrite may not be normally performed possibly.

Therefore, in a semiconductor memory device according to a seventhembodiment of the present invention, the value of clamped current isdetermined for each memory cell MC, in more detail, in accordance withthe address (the distance from the current source to the memory cellMC).

FIG. 25 shows a current limiter circuit operative to control the clampedcurrent with the address.

The current limiter circuit includes a PMOS transistor TR1 operating inthe saturation region, of which output or the set current Iset iscontrolled with the value of current flowing in a PMOS transistor TR2. APMOS transistor TR3 and an NMOS transistor TR4 configure an invertercircuit, which is activated with the pulse voltage Vp for set. An NMOStransistor TR5 has a gate supplied with a bias voltage Vbias, of whichlevel is used to determine the current flowing in the transistor TR2.Therefore, when the bias voltage Vbias is changed in accordance with theaddress, the clamping value of the set current Iset can be changed aswell.

If the pulse height or width of the pulse voltage Vp is stepped up ordown during the repetition of data program and verify as in the sixthembodiment, the above-described clamped current may also be stepped upor down correspondingly.

FIGS. 26A and 26B are diagrams showing such the embodiments.

FIG. 26A provides a graph showing a relation among time, the pulsevoltage Vp6 and the limit value of current flowing in the memory cellMC6.

In the first data program, a certain current limit value is set and thepulse voltage Vp6 is applied. Thereafter, verify is performed to checkthe state of data programmed in the memory cell MC6 and the pulsevoltage Vp6 stepped up or down in accordance with the result is appliedand the clamped current is increased to the extent that is notdestructive of the memory cell MC. Thus, in data program at the nextstep, it is possible to prevent excess current from destructing thememory cell MC and apply larger electric energy than the previous step.

FIG. 26B provides a graph showing a relation among time, the pulsevoltage Vp2 and the limit value of current flowing in the memory cellMC2. The memory cell MC2 is poorer in write characteristic than thememory cell MC6 and requires application of larger electric energy. Evenin this case, the current flowing in the memory cell MC2 is limited toprevent excess current from destructing the memory cell MC2 accordingly.

FIG. 27 is an equivalent circuit diagram of a memory cell and lines in asemiconductor memory device according to yet another embodiment of thepresent invention.

This circuit includes a current limit means or a current clampingtransistor TRc interposed in the bit line BL additionally to theequivalent circuit shown in FIG. 16. This arrangement makes it possibleto limit current flowing in the word line WL, the memory cell MC and thebit line BL in accordance with the base voltage on the transistor TRc.The base voltage on the transistor TRc can be controlled based on theresistance of the variable resistor VR.

[Others]

Although writing is described above, the pulse voltage Vp can becontrolled also on erasing and reading to even characteristics among thememory layers or memory cells.

It can be considered that thermal histories may influence onimprovements in crystallinity and orientation, on reductions in initialresistance, and on improvements in write, erase and read characteristicsin lower layers, in contrast to the cases of FIGS. 8-18.

Even in such the case, the electrical energy fed to the memory cell MCcan be increased at a higher layer to even the characteristics among thememory layers CA or memory cells. Even though there is no regularity invariations in write, erase and read characteristics, parameterscorresponding to the memory layers CA may be prepared previously tocontrol the pulse generator 7 in each of the memory layers CA or memorycells MC to even the characteristics among the memory layers CA ormemory cells MC.

In the above-described examples, any one of the height and width of thepulse voltage Vp is controlled. It is though sufficient to adjust theelectrical energy fed to the memory cell MC in each memory layer CA and,for example, both the height and width of the pulse voltage Vp may becontrolled to exert the effect of the present invention as well.

1. A semiconductor memory device, comprising: a plurality of memorylayers arranged in multilayer, each memory layer including a cell array,said cell array containing a plurality of first parallel lines, aplurality of second parallel lines arranged crossing said first lines,and a plurality of memory cells connected at intersections of said firstlines and said second lines; a pulse generator operative to generatepulses required for data access to said plurality of memory cells; and acontrol means operative to control said pulse generator such that thepulse output from said pulse generator has energy in accordance with thememory layer to which an access target memory cell belongs, wherein saidpulse generator supplies plural pulses different in step width at eachmemory layer to said first or second lines in each of said memorylayers.
 2. The semiconductor memory device according to claim 1, whereinsaid control means controls said pulse generator based on the address ofthe access target memory cell and a parameter previously set at each ofsaid memory layers.
 3. The semiconductor memory device according toclaim 2, wherein said parameter is held in any one of said memory cells.4. The semiconductor memory device according to claim 1, wherein saidpulse generator simultaneously supplies plural pulses different in atleast one of width and height to said first or second lines in each ofsaid memory layers.
 5. The semiconductor memory device according toclaim 1, wherein said pulse generator increases or reduces the energy ofsaid pulse supplied as said access target memory cell nears the aimedphysical state.
 6. The semiconductor memory device according to claim 1,wherein said pulse generator reduces the energy of said pulse suppliedas the memory layer becomes a higher layer.
 7. The semiconductor memorydevice according to claim 1, wherein said pulse generator reduces theenergy of said pulse supplied as the memory layer becomes an upperlayer.
 8. The semiconductor memory device according to claim 1, whereinsaid cell array further comprising a current limiter operative to limitcurrent flowing in said memory cell on access to said memory cell, andsaid current limiter sets the limit value of current in accordance withthe address of the access target memory cell.
 9. A semiconductor memorydevice, comprising: a plurality of memory layers arranged in multilayer,each memory layer including a cell array, said cell array containing aplurality of first parallel lines, a plurality of second parallel linesarranged crossing said first lines, and a plurality of memory cellsconnected at intersections of said first lines and said second lines,each said plurality of memory cells containing an electricallyrewritable variable resistor operative to store the resistance as data;a pulse generator operative to generate pulses required for data accessto said plurality of memory cells; and a control means operative tocontrol said pulse generator such that the pulse output from said pulsegenerator has energy in accordance with the memory layer to which anaccess target memory cell belongs, wherein said pulse generator suppliesplural pulses different in step width at each memory layer to said firstor second lines in each of said memory layers.
 10. The semiconductormemory device according to claim 9, wherein said memory cell furtherincludes a non-ohmic element serially connected with said variableresistor.
 11. The semiconductor memory device according to claim 9,wherein said control means controls said pulse generator based on theaddress of the access target memory cell and a parameter previously setat each of said memory layers.
 12. The semiconductor memory deviceaccording to claim 11, wherein said parameter is held in any one of saidmemory cells.
 13. The semiconductor memory device according to claim 9,wherein said pulse generator simultaneously supplies plural pulsesdifferent in at least one of width and height to said first or secondlines in each of said memory layers.
 14. The semiconductor memory deviceaccording to claim 9, wherein said pulse generator increases or reducesthe energy of said pulse supplied as said certain access target memorycell nears the aimed physical state.
 15. The semiconductor memory deviceaccording to claim 9, wherein said cell array further comprising acurrent limiter operative to limit current flowing in said memory cellon access to said memory cell, and said current limiter sets the limitvalue of current in accordance with the address of the access targetmemory cell.